The objectives of this research program are to characterize the properties of the mucosal membranes, the baso-lateral membranes and the extracellular shunt pathway in in vitro rabbit ileum under absorptive and secretory conditions. Particular emphasis is placed on the elucidation of the characteristics of a neutral coupled NaCl influx process at the musocal membranes which is non-competitively inhibited by an elevation in intracellular cyclic-AMP concentration and appears to be central to normal NA and Cl absorption as well as Na and Cl secretion under pathological conditions (e.g. challenge of the tissue with cholera toxin). In addition, we have recently found that rabbit gallbladder is characterized by a neutral NaCl coupled influx process at the brush border which in many respects resembles that found in rabbit ileum. In view of the fact that unlike rabbit ileum, the influx process in rabbit gallbladder is the only process involved in neutral NaCl transepithelial transport and the fact that rabbit gallbladder is characterized by only one cell type, this tissue appears to be an ideal model for the investigation of the mechanism by which elevated levels of intracellular cyclic-AMP inhibit the coupled NaCl influx process and transepithelial transport.